A CIS solar cell and a CIGS solar cell are thin film solar cells that comprise three elements (Cu, In and Se) and four elements (Cu, In, Ga and Se), respectively. The CIGS solar cells have higher photoelectric conversion efficiency than Si thin-film solar cells, and the related markets are expected to grow in the future. Generally, the CIGS solar cells have a structure such as substrate/Mo electrode/CIGS/CdS/TCO. The substrate is made of, for example, a plastic, glass or metal foil (thin metal substrate) material, and is selected in consideration of suitability for fabrication processes, and properties such as flexibility.
Glass substrates are transparent and are flexible when processed to have a thin thickness, but are difficult to handle. Plastic substrates have low heat resistance, and thus special processes such as low-temperature vapor deposition should be applied thereto.
Meanwhile, in the case in which a solar cell is to be fabricated on a stainless steel foil (thin stainless substrate material) or plastic substrate, when heat is applied to the substrate during vapor deposition or in the thin-film cell layer formation and selenization processes, there may be a problem in that interlayer separation occurs, because the substrate and the CIGS layer have different thermal expansion coefficients.
To overcome this problem, a substrate having a thermal expansion coefficient similar to that of the CIGS layer is preferably used, and for this reason, a specific metal foil may be selected as the substrate material. A metal foil can be rendered flexible when processed to have a thin thickness. In addition, it has high heat resistance that eliminates the need to design a low-temperature deposition process. Accordingly, a substrate made of an alloy material found to have a thermal expansion coefficient similar to that of CIGS will have various advantages.
Various methods can be used to manufacture metal foils, and among them, rolling technology is frequently used. The rolling technology is a technology of processing a metal foil using 20-stage rolling rolls. This technology has limitations in increasing the width of the metal substrate material being processed, and thus is disadvantageous for producing a large-area substrate using a large-area production process with reduced production costs. In addition, it is generally difficult for this technology to obtain a substrate material having a thickness of 0.1 mm or less.